The present invention relates to a driving apparatus which can perform a reproducing and/or recording operation in accordance with an optical recording medium.
CD (compact disc)-type discs, a DVD (digital versatile disc/digital video disc) that is suitable for multimedia purposes, and other kinds of discs have been developed as optical disc recording media.
In driving apparatuses for those kinds of optical discs, data is read out by applying laser light to a disc and detecting resulting reflection light.
For the above readout operation to be performed satisfactorily, it is required that laser Light be in a focused state at the position of the disc recording surface. For such a focusing state control, a focus servo circuit is provided which keeps laser light focused on the disc recording surface by moving an objective lens (which is a laser light output end) in the direction in which it comes closer to or goes away from the disc in accordance with a focus error signal that is produced according to an astigmatism scheme, for example.
To allow the focus servo circuit to effect focus pull-in satisfactorily, it is necessary to detect a portion of a movable range of the objective lens where a good focus error signal (what is called an S-shaped curve) is obtained. Therefore, in a start process that is executed upon disc insertion or in re-establishing a focused state when focusing is lost during a playback or a seek operation, first an operation called "focus search" is performed to detect a focus pull-in range. The focus search is an operation for determining a range (focus pull-in possible range) where an S-shaped curve can be obtained in a focus error signal as the objective lens is moved, for example, from a position that is most distant from the disc to a position that is closest to the disc (i.e., over an objective lens movable ranged as a focus stroke) in a state that a focus servo loop is opened.
By closing the focus servo loop in a state that the objective lens is located in the focus pull-in range, the position of the objective lens can be led to a focusing position that corresponds to the zero-cross point of the S-shaped curve in a focus error signal.
It is preferable that a focus search operation be completed as quickly as possible. One method for satisfying this requirement would be to move the objective lens fast in the focus stroke.
However, moving the objective lens too fast increases a possibility that a focus pull-in attempt fails when a focus servo operation is turned on after a focus pull-in possible range has been detected.
On the other hand, if focusing is lost, for example, during a disc playback (e.g., during a seek operation) that is performed by the disc driving apparatus in response to a read instruction that is sent from a host computer, a focus search operation that lasts too long may cause the host computer to execute an error process with a judgment that a time-out has occurred. A focus search operation that takes long time is not preferable because it delays a transfer of reproduced data to a computer so much, even if a time-out does not occur.
However, it is difficult to increase the speed of the focus search operation is because simply increasing the speed of the focus search operation increases the possibility of a focus pull-in failure as described above.